Nickel-tantalum addition agent for incorporating tantalum in molten nickel systems

ABSTRACT

A physical admixture of nickel powder and scrap tantalum powder is made to dissolve in molten nickel as rapidly as a conventional true master alloy of comparable proportions of these metals in pure form by pressing the admixture of the two metal powders under a pressure of at least 10,000 pounds per square inch.

United States Patent [1 1 Gustison A NICKEL-TANTALUM ADDITION AGENT FORINCORPORATING TANTALUM IN MOLTEN NICKEL SYSTEMS Robert A. Gustison,Reading, Pa.

Kawecki Berylco Industries, Inc., New York, N.Y.

Filed: July 12, 1972 Appl. No.: 271,114

Inventor:

Assignee:

US. Cl. 29/192 R, '75/0.5 R, 75/170, 264/11 1 Int. Cl. B221 3/02, B22f3/04 Field of Search 75/0.5 R, 170, 171; 29/192 R, 420; 264/111References Cited UNITED STATES PATENTS 8/1960 Thielman 75/171 51 Dec.18, 1973 3.161426 l/l965 Freche et a1 75/171 OTHER PUBLICATIONS Goetzel,C. 6.; Treatise on Powder Metallurgy; Interscience Publishers, Inc.;1950 Vol. II; page 634.

Primary ExaminerW. W. Stallard Att0rneyDean S. Edmonds et al.

[5 7] ABSTRACT A physical admixture of nickel powder and scrap tantalumpowder is made to dissolve in molten nickel as rapidly as a conventionaltrue master alloy of comparable proportions of these metals in pure formby pressing the admixture of the two metal powders under a pressure ofat least 10,000 pounds per square inch.

4 Claims, No Drawings NlCKEL-TANTALUM ADDITION AGENT FOR INCORPORATINGTANTALUM IN MOLTEN NICKEL SYSTEMS This invention relates to masteralloys and, more particularly, to nickel-tantalum master alloys foraddition to molten nickel.

in the production of high temperature oxidationresistant alloys such asnickel base alloys containing tantalum, the high melting point oftantalum has made it difficult to dissolve tantalum metal directly intomolten nickel either in producing the desired alloy directly or inproducing a high tantalum-content nickel base master alloy for additionto molten nickel. ,7 I,

It has been proposed heretofore to produce master alloys by briquettingthe individual powders of the metals of the master alloy (GoetzelsTreatise on Powder Metallurgy, lnterscience Publishers, Inc., N. Y.1950, Vol. IL, pages 569-570) but this technique has generally beenlimited to powder components of metals of low to moderate meltingpoints. Where master alloys have been made of components including hightemperature melting point metals such as tantalum, it has been thepractice to produce the desired master alloy by the aluminothermicreduction of tantalum oxide in the presence of nickel powder and oftenwith the addition of a heat-producing supplemental oxidizer such assodium chlorate or barium peroxide (Ibid., page 634). The latterapproach has the following disadvantages when applied to the productionof a nickel-tantalum master alloy:

1. Poor recovery of tantalum (generally between 80 to 90 percent);

2. Variable concentration of tantalum in the master alloy;

3. Residual amounts of the reducing agent (aluminum) in the final alloy;

4. Use of high-priced virgin starting material (tantalum oxide);

5. Variability of alloy particle size because the smelted alloy must becrushed;

6. Narrow range of acceptable tantalum compositions which is based onthe lower melting point ranges of the nickel-tantalum phase diagram.

1 have now found that ideal nickel-tantalum addition agents forincorporating tantalum in molten nickel systems can be produced bypressing an intimate mixture of powders of these metals. The resultingproduct, consisting of an intimate physical admixture of tantalum metalpowder and nickel metal powder compressed under a pressure of 10,000 to100,000 pounds per square inch, and not subsequently sintered, dissolvesin molten nickel or nickel-base alloy at least as fast as a true masteralloy of comparable proportions of nickel and tantalum.

The high rate of solution of the nickel-tantalum addition agent of theinvention is attributable to the uniform intimate contact between itscomponents with resulting promotion of uniform exothermic reactionbetween these components. When a finely divided mixture of nickel andtantalum powders compacted pursuant to the invention is introduced intoa molten nickel bath, the following sequence of steps can be visualizedfor the process of dissolution of the addition agent in the moltennickel:

l. The compact is rapidly and uniformly heated to the temperature of themolten bath;

2. The nickel component of the compact starts to melt;

3. The molten nickel component of the compact reacts with the highsurface area tantalum powder, forming an intermetallic compound such asTaNi and Ta Ni and 4. The nickel-tantalum reaction evolves heat whichassists in the solution of the resulting nickel-tantalum intermetalliccompound in the molten nickel.

The range of nickel to tantalum proportions can vary from 1 to 99percent by weight of either component, the balance being essentially theother component. A proportion of about 35 percent nickel and 65 percenttantalum, carried over from the conventional nickeltantalum master alloyart, can be used with particular advantage.

Both nickel and tantalum metal powders are readily available in highpurity form for use in practicing the invention. Nickel powder isgenerally obtained by the decomposition of nickel carbonyl gas intonickel powder and carbon monoxide, and this form of nickel powder ispresently preferred for use in practicing the invention. Tantalum powderis generally obtained from either of two sources, and it is a particularadvantage of the present invention that it can use a high puritytantalum powder whose cost is reduced by being classed as, or beingderived from, scrap tantalum. One of these sources comprises tantalummetal as normally produced in powder form. Such powder as does not meetthe requirements for tantalum capacitor grade powder is generallyconsidered scrap by such standards, but is nevertheless of high purityand is wholly suitable for use in producing the compacts of thisinvention. The other source of tantalum powder for use in practicing theinvention is from metallurgical tantalum scrap, such as mill ends, sheetor plate croppings, ingot butts and the like, which can be hydrided,ground and dehydrided to produce a high purity tantalum metal powder.Inasmuch as both of these tantalum sources can be considered as scrapsources, the prac' tice of the present invention can be considered ameans of converting tantalum scrap into a useful product.

The particle size of the nickel and tantalum powders is not critical,although the powders are advantageously of minus 60 mesh Tyler standard,and preferably minus 100 mesh, for optimum intimacy of the particles ofthe two metals in the mixture. This intimacy of contact is furtherenhanced by thorough mixing of the two powders for, in general, at least10 minutes in a conventional tumbler-type blender.

By pressing the nickel-tantalum powder mixture at pressures of at leastabout l0,000 psi, and up to about 100,000 psi, and advantageously underisostatic conditions, the particles of nickel and of tantalum arebrought into a uniform degree of contact in all directions and with adegree of intimacy conducive to prompt reaction of the tantalum with thenickel when the nickel particles have been heated to the molten state.This intimate and unifonn contact between the nickel and the tantalumthroughout the mass of the compacted powder also insures uniform andrapid transfer of heat to the interior of the compact from the moltenmass of nickel in which it is immersed during the tantalum innoculationor addition operation.

The practice of the present invention is illustrated by the followingexample:

A mixture of 350 pounds of lnco Type 128 carbonylnickel powder, with anaverage particle size of 7-9 microns, and 650 pounds of minus 100 meshTyler standard tantalum powder prepared by hydriding tantalum scrap,grinding to minus 100 mesh in a ball mill and dehydriding the resultingpowder under vacuum at 950 C., was blended for 15 minutes in aconventional twinshell blender. Polyethylene bags encased in perforatedcopper retaining molds 1-% inches square by about 30 inches long werefilled with the blended powder mixture, were sealed and were thenintroduced into an isostatic press wherein a pressure of 60,000 poundsper square inch was applied. The powder mixture was thus converted intostrong bars l-% inches square by about 30 inches long which were shearedinto 1 to l-Vz inch lengths. The density of the powder metal compactswas about 85-90 percent of the theoretical density of its components. Tocompare the solution rates in molten nickel of the compact of theinvention versus a conventional nickel-tantalum master alloy, threesamples were prepared:

1. A piece of nickel-tantalum compact, produced as described in theforegoing example and weighing 100 grams;

2. A 100 gram piece of the aforementioned compact which had beensintered under vacuum at l,000 C. for 60 minutes; and

3. A 100 gram piece of a conventional 35 percent Ni-65 percent Ta alloyprepared by aluminothermic reduction of Ta O in the presence of nickelpowder.

The solution rate of each of the three samples in the form of a rod wasmeasured in a molten bath of pounds of nickel held at 1,600 C. Theresults were:

Sample No. Solution Rate, grams/second l 5.6 2 5.7 3 5.2

TABLE I Purity of Ni-Ta Compact Vs. Ni-Ta Alloy Element Ni-Ta CompactNi-Ta Alloy Ni 35 i 0.2 35-36 Carbon 0.02 0.02 Hydrogen 0.005 0.00] Iron0.005 0.!

Oxygen 0.l 0.03 Nitrogen 0.005 0.006 Sulfur 0.005 0.007 Phosphorous0.005 0.0! Silicon 0.15 0.07 Manganese 0.005 0.002 Aluminum 0.005 0.50

The nickel-tantalum compacts of the invention have the followingadvantages over standard aluminothermically produced nickel-tantalummaster alloys as addition agents for introducing tantalum into moltennickel systems:

1. Exact composition for exact dosage of added tantalum;

2. Uniform composition;

3. A solution rate in molten nickel at least as fast as that of themaster alloy;

4. High recovery of contained tantalum;

5. Can utilize low-cost tantalum scrap;

6. No cleaning of the alloy is required, as is the case with analuminothermic master alloy;

7. No residual amounts of reducing agent in the product;

8. Higher purity in most respects;

9. Uniform shape of addition agent as produced; and

10. Infinite range of compositions possible.

1 Claim:

1. A compacted powder metal nickel-tantalum addition agent forincorporating tantalum in molten nickel systems which comprises anunsintered intimate physical admixture of nickel metal powder andtantalum metal powder compressed under a pressure of at least about10,000 pounds per square inch.

2. An addition agent according to claim 1 having a density of about topercent of maximum theoretical density.

3. An addition agent according to claim 1 in which the metal powders areof minus 60 mesh particle size.

4. A product in accordance with claim 1 wherein the blended mixture iscompacted isostatically at a pressure of 10,000 to 100,000 pounds persquare inch.

2. An addition agent according to claim 1 having a density of about 75to 95 percent of maximum theoretical density.
 3. An addition agentaccording to claim 1 in which the metal powders are of minus 60 meshparticle size.
 4. A product in accordance with claim 1 wherein theblended mixture is compacted isostatically at a pressure of 10,000 to100,000 pounds per square inch.